My girlfriend is a French citizen. She is separated — not yet divorced from her Mexican citizen husband. How can she become a US citizen?

It is difficult to give an answer as there are not enough facts in your question. Generally speaking, individuals cannot become US citizens under most circumstances unless they are first permanent residents. How your girlfriend could become a permanent resident might depend on many factors, and she may wish to consult with an immigration lawyer to go over her options. For example, if you are a US citizen and if your girlfriend divorces from her husband, you could possibly marry her and petition for her green card. Due to the limitations of the Lawyers.com Forums, Alan Lee, Esq.'s (the "Firm") participation in responding to questions posted herein does not constitute legal advice, nor legal representation of the person or entity posting a question. No Attorney/Client relationship is or shall be construed to be created hereby. The information provided herein by the Firm is general, and requires that the poster obtain specific legal advice from an attorney. The poster shall not rely upon the information provided herein as legal advice nor as the basis for making any decisions of legal consequence.